This invention relates to the processing of fine minerals i.e. minerals which consist substantially completely of particles smaller than 150 microns equivalent spherical diameter.
It is well known to remove impurities from minerals by forming the mineral into a slurry, conditioning the slurry and subjecting the conditioned slurry to a froth flotation process. In general, such a process comprises adding a collector, for example a long chain fatty acid, such as oleic acid, or an aliphatic amine containing from 8 to 20 carbon atoms, to the slurry of the mineral, conditioning the slurry by agitating the slurry in a tank, the conditioning generally being carried out for a time which results in the dissipation in the slurry of up to about 5 horsepower hours of energy per ton of solids, and then passing air through the slurry in a known manner in a froth flotation cell to effect a separation of impurities from the desired mineral. In most cases, the froth flotation process is carried out with the slurry at an alkaline pH and it may be advantageous to add an alkali, for example ammonium hydroxide, to the slurry before the conditioning thereof to ensure that the optimum pH conditions obtain. It is also well known to add a frothing agent, for example pine oil, to the conditioned slurry before carrying out the froth flotation process.
However, in many cases, and especially when treating fine minerals, it is found that the selectivity of the known processes is low, i.e. the proportion of the impurities removed is of the order of 50 percent or less. In addition some minerals, e.g. clays, naturally contain a high proportion, i.e. greater than 40 percent by weight, of particles of extremely small dimensions, i.e. two microns equivalent spherical diameter or smaller, and it is extremely difficult to separate impurities from such minerals by a simple froth flotation process of the type described above because the individual, extremely small particles of the mineral are grouped together into large masses and are flocculated, the impurities being included and held in the flocculated masses. In fact, in many industrial froth flotation processes the material consisting of particles smaller than 50 microns, which material takes the form of a slime, is separated from the crude ore and discarded. In order to try and overcome these problems it has been proposed to deflocculate the fine material, for example by the addition of a dispersing agent, e.g. sodium silicate, to the slurry, before treating the same by a froth flotation process. It has also been proposed to add an activator, for example a water-soluble salt of an alkaline earth metal, to the slurry before carrying out the froth flotation process. However, such processes still do not give wholly satisfactory results with slimes, and more recent attempts to try and beneficiate such materials have involved the use of auxiliary carrier particles which effectively increase the size of the mineral particles; processes of this type are disclosed, for example, in U.S. Pat. Specifications Nos. 2,990,958 and 3,224,582. However, these processes suffer from the drawback that they increase the cost of the product substantially and also introduce the risk of contaminating the desired mineral with auxiliary carrier particles thus necessitating a further separation step if the desired mineral is required in a pure state. This latter drawback arises because there is always the danger that some of the auxiliary carrier particles will not be sufficiently hydrophobic to float when introduced in the froth flotation process, even though the auxiliary carrier particles are intended to report to the froth together with the unwanted particles.
The prior art froth flotation processes described above have been carried out in a variety of types of froth flotation cell; one type of froth flotation cell which is frequently used comprises a central, internal, submerged impellor which is rotated at peripheral speeds up to about 1250 feet/minute.